The present invention relates to a rotary encoder for detecting an angle, and more particularly to an absolute encoder which can read a pattern even when rotating at high speed.
At present, an absolute type and an incremental type are employed for optical encoders that are used for a surveying instrument.
An encoder employing the incremental type converts the number of count from a zero position into an angle. The conversion into an angle requires detection of a zero position.
The absolute type encoder is of a type in which an angle value is in a one-to-one correspondence with a position on the circumference of a circle. The absolute type encoder is characterized in that since a position on the circumference is registered as an absolute address, position information can be obtained at all positions.
Thus, if the absolute encoder is used, position information can be obtained at all positions because counting is not required. However, there was the following problem because it takes time to detect and process a signal of an absolute pattern, it is disadvantageous to dynamic reading.
When an operator himself/herself conventionally operates a surveying instrument to perform surveying work, reading is basically performed after directing the surveying instrument toward a collimation direction. Therefore, even when it is rotated as fast as possible, if it is stopped finally and reading is performed in a stationary state, no problem will arise. However, there is a problem in the case of an automatic surveying instrument that automatically turns toward a target for collimation or continuously tracks a target; recently, the automatic surveying instrument is frequently used.
To be more specific, the automatic surveying instrument to which a tracking function is added continues tracking to measure a position of the target. Under the circumstances, the incremental encoder is suitable for constrained operation to some extent. However, once count is missed, it is impossible to measure an angle unless a zero position is detected again. Therefore, the incremental encoder is not suitable for such use, which is the problem.
In the absolute type encoder disadvantageous to reading during operation, when the encoder rotates at high speed while detecting an absolute pattern, the contrast of a signal waveform becomes worse, resulting in a decrease in accuracy and impossibility of reading. Therefore, it is necessary to decrease a rotational speed of the encoder to a certain level. Accordingly, there is a problem that the absolute encoder cannot read a measuring target that is continuously moving at high speed.
In addition, even if a measuring target is moving at low speed that allows measurement, a gap between a read measured value of the absolute encoder and a real position is caused by time difference in detection processing, which is another problem.
The present invention relates to a rotary encoder for detecting an angle, and to an encoder reading device that projects a pattern formed on an encoder onto a light receiving means using light emitted from a light source means so as to read a position of the encoder. The light source means is configured to emit a pulse that is synchronized with a shift pulse which moves an electric charge to a transfer gate of the light receiving means, and that is sufficiently shorter than a time interval (accumulation time) of the shift pulse.